1. Field of the Invention
This invention relates to heat resistant copolymers having a high content of .alpha.-methylstyrene, a process for their production and thermoplastic resin compositions which contain .alpha.-methylstyrene high-content copolymers and have excellent heat resistance and impact resistance.
2. Description of the Prior Art
Today, a great number of thermoplastic resins are being utilized. One representative example thereof is ABS resins which are rubber-modified thermoplastic resins. However, in the field where high resistance to heat distortion was required, ABS resins were inadequate in this respect. Various methods to enhance the heat resistance of ABS resins have been proposed. For example, Japanese Patent Publication No. 18194/1960 describes that a composition having high heat resistance and impact resistance is obtained by mixing a copolymer comprising .alpha.-methylstyrene and acrylonitrile with an ABS resin. However, the heat distortion temperature of this composition is as low as slightly above 100.degree. C., and therefore its use has been inevitably limited where higher heat resistance was required. This is because when .alpha.-methylstyrene and acrylonitrile are free-radical polymerized in the emulsion state, the resulting copolymer is, for the most part, an alternating copolymer even when the ratio of both reactants to be charged is changed. That is, when the amount of .alpha.-methylstyrene is 70% by weight or more, the conversion to polymer suddenly decreases and .alpha.-methylstyrene monomer is thus left unreacted. This tendency is also observed with a terpolymer comprising .alpha.-methylstyrene, acrylonitrile and styrene. Further, Japanese Patent Publication No. 33661/1970 describes a process which comprises introducing .alpha.-methylstyrene into a polymer. According to this process, 75-90% by weight of .alpha.-methylstyrene and 25-10% by weight of acrylonitrile are copolymerized in a monomer mixture in the first stage, and subsequently in the second stage the polymerization of this gradually remaining .alpha.-methylstyrene is completed by adding a monomer chiefly comprising styrene and acrylonitrile. However, by this process, since even when the maximum amount, i.e. 90% by weight, of .alpha.-methylstyrene is employed, a reduction in conversion to polymer is brought about and thus it is difficult to make the content of .alpha.-methylstyrene in the polymer produced in the first stage 82% by weight or higher. Moreover, when the amount of .alpha.-methylstyrene used in the first stage is increased, the amount of the remaining .alpha.-methylstyrene is accordingly increased, which in turn requires an increase in amount of styrene and acrylonitrile to be added to polymerize the above .alpha.-methylstyrene in the second stage, thereby the .alpha.-methylstyrene content in the total polymer eventually results in merely 75% by weight or so. Thus, by the above-described conventional process, the heat resistance of each obtained composition had its limit. Similarly, poly-.alpha.-methylstyrene is known as an .alpha.-methylstyrene high-content polymer, but its thermal decomposition temperature is so low that it cannot be served for practical applications. Also from the aspect of production, it can only be produced by anionic polymerization and cannot be produced by free-radical polymerization.
As the result of our extensive study, we have discovered that an .alpha.-methylstyrene-acrylonitrile copolymer having a certain range of composition is excellent in heat resistance and that by mixing a graft copolymer with this copolymer satisfactory impact resistance is imparted and, based on this discovery, we have finally accomplished this invention.